The invention concerns a tire with radial carcass reinforcement, covered radially by a crown reinforcement consisting of at least two working plies formed of reinforcement elements mutually parallel within each ply and crossed over from one ply to the next so as to make angles of small absolute value ranging from 10xc2x0 to 45xc2x0 with respect to the circumferential direction.
In a tire corresponding to the present state of the art, the carcass reinforcement is formed of at least one ply of reinforcement elements which are radial or are termed radial, in other words elements which make an angle of 90xc2x0xc2x110xc2x0 with respect to the circumferential direction. The said reinforcement elements are either textile cables, which is generally the case in tires for passenger vehicles, aircraft or agricultural machinery, which have one or more carcass plies, or metallic cables in the case of tires for heavy road vehicles or civil engineering machinery, which generally have just one carcass ply. The said elements extend continuously from one bead-wire to the other and are anchored in each tire bead by being turned up around the bead wire.
During the manufacture of the tire, the ply or plies of the carcass reinforcement, whether of textile or metal, such as steel, are first positioned on a cylindrical drum and then shaped to form a toroidal tire blank before being radially capped with a crown reinforcement. During molding in the mold of the vulcanizing press, the toroidal tire blank so obtained undergoes additional shaping.
In the case of a tire with metallic carcass reinforcement, whose reinforcement elements are nonextensible, i.e. showing very little elongation under a tensile force equal to 10% of the breaking force, the various transformations undergone by the unvulcanized carcass and tire blanks give rise to a phenomenon known as flowing of the internal layers of rubber mix, particularly in the area of the shoulders of the tire. This flowing phenomenon varies circumferentially in its extent and is or can be an important factor in reducing the endurance of the carcass reinforcement of the tire. In the case of tires with radial carcass reinforcement consisting of reinforcement elements made of a thermo-contracting textile or plastic material, the vulcanization of the tire leads to the same phenomenon of flowing of the internal layers.
An obvious solution for resolving such problems would be to increase the thickness of the inside layers considerably, but equally obviously, such a solution would not be optimum either from the standpoint of the tire""s properties (since the extra thickness would result in more heating at the shoulders, increased rolling resistance, etc.) or from that of manufacturing cost.
Patent FR 1 111 805 describes a carcass reinforcement in two distinct portions, which overlap in the area of the tire""s equatorial plane. This way of arranging the ply or plies of the carcass reinforcement has technical advantages, both as regards ease of manufacture and as regards the flexibility of the tire.
Patent FR 1 111 806 describes the same arrangement of two portions of the radial carcass reinforcement to make up for the shortcomings of the triangulation ply located radially above the working crown plies.
Patent FR 1 425 801 recognizes that this arrangement of carcass ply layers which are not continuous but superimposed in the equatorial portion of the tire not only affords the desirable reinforcement under the tread but also provides a better balance of forces during the shaping of the unvulcanized tire blank because the overlap in the equatorial area of the tire allows certain relative movements of the resistance elements before vulcanization.
Such movements are not sufficient to avoid the phenomenon of flowing of the internal layers through the carcass reinforcement cables because the contact width between the two overlapping portions, an overlap which is necessary to ensure anchoring by shearing of the two portions, is too large since it is generally almost equal to the width of the crown reinforcement. Moreover, the radial superimposition of several carcass layers underneath the crown reinforcement influences the meridian flexibility of the carcass reinforcement - crown reinforcement assembly, and in particular the position of the neutral bending axis, which is not always desirable.
The invention proposes a different solution, which consists in subdividing the carcass reinforcement that is continuous from one bead-wire to the other into three parts, and providing the part of the carcass reinforcement under the crown reinforcement with elasticity greater than the elasticity of the parts that reinforce the sidewalls of the tire, this greater elasticity being achieved by the use of cut reinforcement elements.
According to the invention, a tire comprising a carcass reinforcement consisting of at least one ply of reinforcement elements anchored in each tire bead to at least one bead-wire, the said carcass reinforcement being radially covered by a crown reinforcement consisting of at least two working plies formed of reinforcement elements crossed over from one ply to the next and making angles between 10xc2x0 and 45xc2x0 with respect to the circumferential direction, is characterized in that the part of the carcass reinforcement underneath the crown reinforcement, centered on the equatorial plane and having axial width between 0.4 and 1 times the width of the widest working crown ply is formed of cut reinforcement elements.
The preferred solution consists in that each reinforcement element of the part of the carcass reinforcement whose width is between 0.4 and 1 times the axial width of the widest working crown ply is cut at least once.
Advantageously, in the unvulcanized condition the said portion under the crown reinforcement will have a linear tensile rigidity per unit width between 0.1 and 0.7 times the linear tensile rigidity of each part of the same carcass reinforcement which reinforces the sidewall of the tire, such that the said ratio of linear rigidities becomes between 0.2 and 1 when the tire is vulcanized.
A rigidity before vulcanization of the portion under the crown lower than 0.1 times the rigidity of the sidewall portion would make it impossible to ensure geometrical stability of the uncured tire blank and, in the vulcanized condition, would not allow sufficient tension under the effect of the inflation pressure to ensure satisfactory triangulation of the crown reinforcement. A linear rigidity greater than 0.7 times the rigidity of a sidewall portion would not avoid flowing of the internal rubber layers.
The linear tensile rigidity results from the tensile force exerted on a strip of the carcass reinforcement, of unit width, required to obtain a relative elongation xcex5 of 0.5%. It can be expressed by the formula R =dF/dxcex5 where R is the linear rigidity, and dF/dxcex5 is the derivative of the force per unit width with respect to the relative elongation xcex5. In the case of the linear rigidity in the unvulcanized condition, it is measured on a strip of the semi-finished product, namely the unvulcanized carcass reinforcement. In the case of the linear rigidity in the vulcanized condition, it is measured on a strip of unit width cut from the vulcanized tire.
Whatever the solution adopted to obtain such linear rigidity ratios, the tension of the reinforcement elements of the carcass reinforcement in the sidewalls of the tire will become almost constant all round the circumference of the tire. Besides the fact that this solves the problem of interior layer flowing on which the invention is based, the solution of cut reinforcement elements makes it possible for the carcass reinforcement in part to preserve its role as a triangulation reinforcement, since the cut elements offer some resistance to compression forces. This solution is the most reliable and industrially the least costly, and it also has a number of important advantages, such as giving better uniformity and in particular appreciably reducing the amplitude of the radial variation no matter which harmonic is considered, and therefore also increasing comfort, or such as reducing the deformations of the external surfaces of the sidewalls, deformations which are perceptible to the eye and which confer a faulty-looking external appearance upon the tire.